WO 00/72821 describes apparatus and methods for generating a foam for treatment of varicose veins. In one of the embodiments described in that patent application, foam is generated by pressurising a sclerosant liquid and a physiological gas in a canister and releasing the mixture through a mesh whereby a stable foam is produced suitable for injection into varicosed blood vessels in sclerotherapy treatment. An apparatus is described which incorporates a three-way valve attached to the outlet of the foam generating canister. The first portion of foam generated by the canister tends to be of poor quality, and the valve allows this foam to be diverted to waste. The valve may then be switched over feed foam to a syringe for use in treatment. The disclosure of WO 00/72821 is incorporated herein by reference.
A number of technical problems have been identified in the course of further development of the canister based system described in WO 00/72821. These include the following.
1. When the three-way valve is switched from the waste position to the fill position, there is a momentary dead time when the valve is closed to both outlet ports and flow is completely obstructed. When the valve is fully set to the fill position and the flow re-commences, the foam is initially of poor quality; the canister has effectively re-started its delivery of foam.
2. In a delivery device, such as a syringe, for administering foam to a patient, there is normally a dead space. In the case of a syringe, this is within the bore of the luer connector on the syringe. As foam is directed from the valve to the syringe and pushes the plunger of the syringe back, a large bubble tends to form adjacent the plunger, which may become incorporated within the foam and undermine its quality.
3. It is desirable to be able to inspect the foam and to determine when consistent, good quality foam is being generated, so as to check that foam with the correct properties is being directed into a patient's vein. When foam of consistent quality is being produced, the valve can be adjusted so that foam is directed to the syringe rather than to waste. In the apparatus described in WO 00/72821, the foam is observable in a transparent tube communicating between the canister and valve unit (ref 35 in FIGS. 10 and 11). A difficulty with this is that the foam which is observed is some way back from the foam being delivered. Therefore it is possible to observe adequate quality foam in the tube and still be delivering inadequate quality foam to the syringe.
4. The waste foam from tube 38 is not contained.
5. The use of a relatively long tube 35 joining the canister to the valve is wasteful, since a quantity of foam sufficient to fill the tube will always be wasted.
6. The system is somewhat dependant on the skill of the operator to consistently produce a syringe full of good quality foam.
Although these problems have been discussed above in connection with the system described in WO 00/72821, they may be applicable to other systems for generating and dispensing foam of various sorts, where a uniform foam product having consistent, predetermined properties is required. The contents of WO02/41872, which also relates to this field, is incorporated herein by reference.
A number of solutions to some or all of these problems have been devised by the inventors, as follows.